1. Field of the Invention
The present invention relates to an insulated gate semiconductor device and a method of manufacturing the same. More specifically, the present invention relates to an insulated gate semiconductor device with a trench structure for reducing a gate-drain capacity and to a method of manufacturing the same.
2. Description of the Related Art
FIG. 15 is a cross-sectional view of an insulated gate semiconductor device with a conventional trench structure. By way of example, this drawing shows an n-channel type MOSFET.
A channel layer 24 is provided on the surface of a drain region 22 which is provided on a semiconductor substrate 21. Trenches 27 are then formed in such a way that they penetrate through the channel layer 24. The inner walls of the trenches 27 are covered with a gate oxide film 31, and gate electrodes 33 are embedded therein. Source regions 35 and body regions 34 are formed on the surface of the channel layer 24 to form a source electrode 38. This technology is described for instance in Japanese Patent Application Publication No. Hei 11-67787.
Moreover, an approach has been made to form a thick oxide film on the bottom of a trench in order to reduce a capacity in an insulated gate semiconductor device with such a trench structure.
FIG. 16 shows an example of a technology for making the oxide film formed on the bottom of a trench thicker than the oxide film formed on the side walls of the trench.
Nitride films are provided on the inner walls of trenches TR 21 provided in substrates 56 and 57. The nitride film formed on the bottom of each trench is then removed while remaining nitride films NL41 and NL42 formed on the side walls of the trench intact (FIG. 16A). Thereafter, an oxide film OL52 is selectively allowed to grow on the bottom of each trench where the substrate is exposed (FIG. 16B). The nitride films NL41 and NL42 formed on the side walls of each trench are removed, and gate oxide films GL61 and GL62 are then formed on the side walls of each trench (FIG. 16C). This technology is described for instance in Japanese Patent Application Publication No. 2003-158268.
In addition, FIG. 17 shows an example of a technology for forming a thick oxide film on the bottom of a trench by CVD method.
After forming trenches 107 on a substrate 101, the trenches 107 are completely filled with an oxide film by CVD method. A part of the oxide film is then removed by dry etching or wet etching. In this way an embedded oxide film 110 with a thickness of, for example, about 2,000 Å is formed on the bottom of each trench 107. Thereafter, a gate oxide film 111 with a thickness depending on a drive voltage is formed on the inner wall of each trench 107. Thus, the thin gate oxide film 111 is formed on the side walls of each trench 107 to come in contact with a channel layer 104, and the thick, embedded oxide film 110 is formed on the bottom of each trench 107. This technology is described for instance in Japanese Patent Application Publication No. 2001-274397.
In insulated gate semiconductor devices with a trench structure as represented by MOSFETs, extremely thin insulating films are formed on the inner wall of trenches as the capabilities of such devices expand. Meanwhile, an input capacity Ciss, an output capacity Coss and a return capacity Crss are of importance for MOSFETs and, therefore, it is imperative that they are reduced in order to increase device characteristics.
In particular, a gate-drain capacity Cgd is responsible for the input capacity Ciss, the output capacity Coss and the return capacity Crss. In MOSFETs with a trench structure, the gate-drain capacity Cgd is the capacity of the bottom of a trench. For this reason, an approach has been made to form, by means of an enhanced oxidation or a selective oxidation as described above, a thick oxide film only on the bottom of trenches while providing the side walls of the trenches with a thin oxide film.
However, the technology shown in FIG. 16 requires a process for forming nitride films, a process for removing only a nitride film provided on the bottom of trenches, a selective oxidation process, a process for removing a nitride film provided on the side walls of trenches and a processe for forming a gate oxide film. Thus, there have been problems, for example, that the number of processes is increased and the processes become complicated.
Meanwhile, when oxide films are intended to be embedded in trenches by CVD method or the like as shown in FIG. 17, a hollow space called void or seam is likely to be formed in the oxide films embedded in the trenches. The generation of voids and seams is attributed to the formation of growing seeds on the side walls of trenches in a CVD process. In this case, if there is a portion on the side walls where the seeds grow at a higher rate, an oxide film formed at the portion closes the trench and thereby voids are generated in the oxide film below the portion. In addition, even when voids are not generated, seams are likely to be generated because the oxide films growing from the side walls of a trench are connected at the center of the trench. An etchant penetrates the oxide film through voids and seams and causes the occurrence of abnormal formation, causing a problem that devices are not stably formed.
Furthermore, the following method is also known: forming a thick oxide film only on the bottom of a trench by an enhanced oxidation of a semiconductor layer in which an impurity concentration is selectively increased. However, since this method cannot provide an increased amount of oxide films, it is less effective than the methods for embedding insulating films such as oxide films in trenches.